Passive stream regulating system applicable to heat exchangers

ABSTRACT

A passive stream regulating system applicable to air-flue gas or water-flue gas heat exchangers, preferably natural gas balanced flue or natural flue home heaters and home boilers, which allows to attain non-over oxygenated stoichiometric mixture conditions. The system is included by small diameter tubes or other geometric systems formed by small channels, to produce a strong laminarization of the aerodynamic flow established within the venting tube, which are located in sections of the conduit or passage of incoming cold air to the combustion chamber of the exchanger or in sections of the conduit or passage of hot flue gases released to the environment.

TECHNICAL FIELD

This invention relates to a ‘Passive system for regulating the applicable flow in heat exchangers’, preferable air-flue gas or flue gas-water, preferably natural gas balanced flue or natural vent home heaters and home boilers, and water heaters; which modifies the total aerodynamic restriction of the flue gas circuit upon changes produced in the gas or flame stream, increasing significantly the same when the gas stream decreases. This change is made without direct or indirect action of any restriction mechanism of the passage section, as in the case of a valve. The object of this increment of the aerodynamic restriction, made passively and based on aerodynamic principles known in the previous art, is to regulate the combustion air stream, in order to keep a ratio of the mixture gas-air close to the stoichiometric ratio, as opposed to present heaters, wherein the over oxygenation is higher under a minimum flame condition than under a maximum flame condition, due to the excessive air stream established in the venting system, because of not having a regulating mechanism.

The present device allows keeping high efficiencies under different configurations of use, different vents and powers, due to the use of a totally passive system for regulating the chimney gases, which comprises according to one possible embodiment tubes of small diameter that preferably are located in the inlet cold air conduit or passage to the combustion chamber of the exchanger, in balanced flue heaters (with two tubes, one of cold air and the other of hot gases) or in the hot gas tube in single tube natural vent heaters.

The invention either is equally applicable to natural vent heaters, natural gas or bottled gas operated.

BACKGROUND ART AND ADVANTAGES OVER SAME

Being the temperature of the flue gases released outside very high and in the absence of venting regulation (aerodynamic circuit formed by the aspired air from the environment and the flue gases released to the same), the gas stream within the same is established according to known flow dynamic and thermal phenomena of natural convection in conduits.

Present home commercial heaters do not have devices allowing to regulate freely the air stream admitted over the vent, to keep a gas-air stream ratio close to the stoichiometric ratio required by the chemical combustion reaction (1 in the case of methane gas, main component of the natural gas) upon different conditions of use (maximum or minimum flame) and/or different configurations of use (horizontal vent, balanced flue, or vertical vent, TBU).

CH₄+2O₂ èCO₂+2H₂O   (1)

Moreover, it is desirable that combustion is established at all times according o a ratio close to the stoichiometric ratio, as an excessive air stream will cause a decrease in the temperature of the combustion chamber, and consequently, an efficiency loss of the device. It is noted that this kind of regulation is usual in the previous art in case of big industrial boilers, by gas monitoring systems and controlled actuation valves. The characteristics of heaters and other home devices (low cost and ease of operation, etc.) do not allow implementing this type of solutions known in the previous art.

In the assayed commercial heaters, it has been found that the air stream imposed in operation to the vent was several times greater than the minimum required by combustion (stoichiometric), being this excess greater under minimum flame conditions than in maximum flame conditions. This is because dimensioning of the venting system (tubes with a diameter of 3 inches) has been calculated to ensure the necessary air stream under a maximum gas stream condition, that is to say, maximum flame. Moreover, under minimum flame, this system is irremediably over dimensioned, causing an excessive air stream, as it cannot regulate the same. This condition can be explained from a fluidynamic standpoint, as due to dominant pressure losses in the venting circuit are those concentrated in the geometry (tortuous) of the combustion chamber. As broadly known in engineering, the pressure jump caused in a medium density flow r and average speed V inside a tubing of D diameter can be calculated for concentrated losses as (2):

DP=½K*r*V²   (2)

wherein DP=total pressure jump in the tubing and K is the total coefficient of localized frictions, that is constant upon changes of the flow rate. In this case, of flow dominated by natural convection, the total DP of the venting is imposed by the density differences between the cold branch (inlet air) and the hot branch (flue gases). The experimental evidence collected lets us determine that it would be convenient, for the previous art commercial heaters, to adjust the friction coefficient K by an increase upon a decrease of the gas stream delivered, which is directly related to the flow rate V, in (2). This would be done, for example, restricting the passage section in a valve, but not in the present devices.

In addition, it has been observed in the previous art that in high vertical chimney configurations (known as TBU) it is established a combustion air stream superior than in a TB system (horizontal, less venting height), strongly reducing the efficiency of the heater on the same grounds that in the previous case. Consequently, regular thermal efficiencies (40-60%) are observed, strongly depending from the type of chimney and the gas stream operated.

This excessive stream is the cause of regular efficiencies, as previously noted. Specifically, this phenomenon is strongly stressed when a heater designed for gas concentric output (low venting) for using indoor (vertical tube output or TBU) wherein there will be a venting of several meters high.

A heater designed to keep a gas ratio close to the stoichiometric ratio under a TB configuration of low venting height could be adapted to be used in higher venting configurations (such as vertical o TBU) by limiting the venting stream inserting a simple orifice plate inside the air tube. However, this solution known in the previous art would not allow regulating the air stream conveniently upon minimum flame conditions, as the added restriction should be calculated for the maximum stream condition, under maximum flame. Again, an adjustable opening orifice should be used, such as that produced by a valve.

This type of high vents (TBU) will be ideal to apply the passive stream regulation system of this invention.

Therefore, it is the main object of the invention to obtain a suitable stoichiometric mixture either at minimum as maximum flame, a characteristic not shown by any of the heat exchangers or heaters known.

To attain the object proposed in the present invention of incrementing the aerodynamic restrictions upon a reduction of the flame gas stream, this invention provides new distributed friction aerodynamic restrictions along the wall of multiple tubes of very small diameter, inserted preferable inside the 3′ external cold air inlet tube. Known plastic tubes as straws have been used, which cost is negligible, being said tubes located in the whole or part of the cold air section of the heater.

This preferred embodiment worked adequately allowing to observe a reduction of over venting at minimum power, and being very affordable, can be easily adapted to different venting heights (that is to say with different streams) by varying the number of ‘floors’ or ‘straws introduced’, (in our experience, up to three are enough).

In case of heaters with concentric gas outlet, the proximity between the inlet tube ‘cold’ and the outlet tube ‘hot’ would prevent from using this material due to the sheet temperature, so the thin or small straws shall be metallic, preferable made of aluminum paper.

The object of this system is to adapt in a simple and configurable way, by using a plurality of internal channels formed by different floors of little tubes and/or using little tubes of different diameter, the total aerodynamic restriction of the gas venting system, so that it is possible to adjust the maximum flame gas stream to a value close to the stoichiometric ratio (optimum), allowing also in a totally passive manner (by using known fluidynamic principles), to increase the aerodynamic restriction coefficient of this system upon a minimum flame usage condition of the heater, wherein it can strongly reduce the gas stream established in the venting system, to bring it close to the optimum value of gas stoichiometric ratio.

By this invention, the dominant aerodynamic losses in the venting flow are those due to distributed frictions (instead of the concentrated losses of the previous art), and so the equation that governs this flow is (3):

DP=½f*(L/Dh)*r*V²   (3)

wherein: L is the total length of the venting system,

Dh is the hydraulic diameter of the passing tubes (5 mm or similar) and

f is the Darcy friction coefficient, given by the equations (4) for smooth wall tubes:

f=64/Re if flow is laminar, typically, if Re<3,000 (4a)

f=0.184 Re ^(−0.2) if flow is turbulent (if Re>3,000) (4b)

wherein Re is the adimensional Reynolds number, given by (5)

Re=V*Dh*r/n   (5)

wherein n is the cinematic viscosity of the fluid, measured in m²/s.

Equations (5) and (4) show that a reduction of the gas stream (by reducing speed, and from that point Re) produces an increase of the friction coefficient f (unlike coefficient K), a feature known in engineering. As shown by equations 4a-b, f varies very slightly at turbulent regime and inversely proportional at laminar regime.

Upon changing the geometry of the air admission tube, from one of about 75 mm of diameter to a plurality of 5 mm, not only a 25 fold increase of the distributed friction values is attained (by changing the ratio L/D), turning into dominant frictions. In addition, it is possible to go from a turbulent flow (in the big diameter tube D) to one strongly laminar (inside the little tubes of diameter Dh), accomplishing other beneficial effect: going from an essentially flow indifferent coefficient of friction, to one inversely proportional to the venting stream.

This last condition constitutes the essence of operation of our passive regulation system: going from maximum to minimum flame allows reducing conveniently the venting stream, by increasing the aerodynamic restrictions of the venting circuit, which causes an operation condition closer to the optimum condition, combustion almost stoichiometric, keeping in this way a high efficiency.

In the tests performed with the usual commercial heater, the efficiency at minimum flame was lower than at maximum flame (due to the phenomenon of absence of venting regulation above described), while with the passive regulation system it was possible to maintain and even increase efficiency at minimum flame condition in relation with maximum flame condition.

DESCRIPTION OF DRAWINGS

In order to understand better the object of this invention, the preferred embodiment of the invention is illustrated by schematic drawings, which are only exemplary, wherein:

FIG. 1 shows a section view of a concentric hot-cold gas tube heat exchanger wherein the cold tube is the inner tube, over which the venting passive regulator of the present invention is installed;

FIG. 2 shows section A-A of FIG. 1;

FIG. 3 is a section view of concentric hot-cold gas tube heat exchanger wherein the cold air conduit is several meters high.

In the drawings, the same references denote identical elements of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In order to understand better the object of this invention, the preferred embodiment of the invention is illustrated by schematic drawings, which are only exemplary, wherein:

FIG. 1 shows a section view of a concentric hot-cold gas tube heat exchanger wherein the cold tube is the inner tube, over which the venting passive regulator of the present invention is installed;

FIG. 2 shows section A-A of FIG. 1;

FIG. 3 is a section view of concentric hot-cold gas tube heat exchanger wherein the cold air conduit is several meters high.

In the drawings, the same references denote identical elements of the invention. 

1. A passive venting regulating system applicable to heat exchangers, of the type comprising at least one conduit or passage of hot gases from the combustion chamber to the environment or having an air inlet conduit to the combustion chamber, characterized in that at least in one section of one of both conduits where said cold air conduit locates, a plurality of tubular elements of low diameter, real or hydraulic, is positioned in parallel, that strongly increase the aerodynamic losses and produce a strong laminarization of the aerodynamic flow circulating though the same.
 2. A passive stream regulating system applicable to air-flue gas heat exchangers according to claim 1, characterized in that said elements are plastic straws.
 3. A passive stream regulating system applicable to air-flue gas heat exchangers according to claim 1, characterized in that said plurality of tubular elements is located at three sections of the cold air conduit.
 4. A passive stream regulating system applicable to air-flue gas heat exchangers according to claim 1, characterized in that the tubular elements have a circular cross-section.
 5. A passive stream regulating system applicable to air-flue gas heat exchangers according to claim 1, characterized in that the tubular elements are made of aluminum.
 6. A passive stream regulating system applicable to air-flue gas heat exchangers according to claim 1, characterized in that the tubular elements are formed by a metallic tube repeatedly wounded over itself, to form between its parallel sides a narrow helical channel. 